1. Field of the Invention
The present invention relates to a technology for replacing a filter that collects soot and solid content by filtering a CO2-absorbing solution used to remove CO2 or the like from exhaust gas, and for cleaning a filtration membrane apparatus.
2. Description of the Related Art
In recent years the greenhouse effect due to CO2 has been pointed out as one of causes of the global warming, and a countermeasure against it is urgently required internationally to protect global environment. CO2 sources range various fields of human activities, including burning of fossil fuels, and demands to suppress their CO2 emission from these sources are on constant increase. In association with this, people have energetically studied methods for suppressing CO2 from power generation facilities such as power plants which use an enormous amount of fossil fuels. One of the methods includes bringing combustion exhaust gas of boilers into contact with an amine-based CO2-absorbing solution, which allows removal and recovery of CO2 from the combustion exhaust gas. Another one of the methods includes storing recovered CO2 without being emitted to the atmosphere.
A method disclosed in Japanese Patent Application Laid-open No. H05-245339 is adopted as the method of removing and recovering CO2 from combustion exhaust gas using the CO2-absorbing solution. The adopted method includes a process of causing the combustion exhaust gas to come in contact with the CO2-absorbing solution in an absorption tower, a process of heating an absorbing solution having absorbed CO2 in a regeneration tower, releasing CO2, and regenerating the CO2-absorbing solution, and again circulating the regenerated CO2-absorbing solution to the absorption tower where the regenerated CO2-absorbing solution is reused.
As shown in FIG. 8, a conventional CO2 recovery system 1000 cools CO2-containing exhaust gas 1001A that contains CO2 exhausted from industrial facilities, such as boilers and gas turbines, in a cooling tower 1002 using cooling water 1003. The cooled CO2-containing exhaust gas 1001A is caused to countercurrently contact an alkanolamine-based CO2-absorbing solution 1005 in an absorption tower 1004, so that CO2 is removed from the CO2-containing exhaust gas 1001A. In a regeneration tower 1007, CO2 is released from a CO2-absorbed solution (rich solution) 1006 that has absorbed CO2. When the CO2-absorbed solution 1006 reaches a lower part of the regeneration tower 1007, most of CO2 is removed from the rich solution 1006 and the CO2-absorbing solution 1005 is regenerated as a lean solution 1008. The regenerated CO2-absorbing solution (lean solution) 1008 is supplied again to the absorption tower 1004 where the regenerated CO2-absorbing solution 1008 is reused as the CO2-absorbing solution 1005.
In a CO2 recovery method using the conventional CO2 recovery system 1000, the CO2-containing exhaust gas 1001A is caused to countercurrently contact the CO2-absorbing solution 1005 that is supplied from a nozzle 1011-1 being a liquid disperser in a CO2-recovery unit 1010 provided in the lower part of the absorption tower 1004, and CO2 in the CO2-containing exhaust gas 1001A is absorbed into the CO2-absorbing solution 1005 by, for example, chemical reaction (R—NH2+H2O+CO2→R—NH3HCO3).
CO2-removed exhaust gas 1001B after CO2 is removed from the CO2-containing exhaust gas 1001A is caused to come in gas-liquid contact with condensed water 1014-1 that contains the CO2-absorbing solution 1005 supplied from a nozzle 1011-2 in a first washing unit 1013-1, and the CO2-absorbing solution 1005 accompanying the CO2-removed exhaust gas 1001B is recovered. The condensed water 1014-1 collected by a first condensed-water receiver 1015-1 is supplied to the first washing unit 1013-1 through a first circulation line L1, and is reused as cleaning water in the first washing unit 1013-1.
Likewise, in a second washing unit 1013-2, CO2-absorbing-solution removed exhaust gas 1001C is caused to come in gas-liquid contact with condensed water 1014-2 that contains the CO2-absorbing solution 1005 supplied from a nozzle 1011-3, and the CO2-absorbing solution 1005 remaining in the CO2-absorbing-solution removed exhaust gas 1001C is recovered. Further, the condensed water 1014-2 collected by a second condensed-water receiver 1015-2 is supplied to the second washing unit 1013-2 through a second circulation line L2, and is reused as cleaning water in the second washing unit 1013-2.
Thereafter, the CO2-absorbing-solution removed exhaust gas 1001C from which the CO2-absorbing solution 1005 is removed is discharged from the top of the absorption tower 1004.
The rich solution 1006 is heated by the lean solution 1008 in a rich-lean solutions heat exchanger 1016 and is supplied to the regeneration tower 1007 through a rich-solution supply line 1017. The rich solution 1006 supplied into the regeneration tower 1007 releases most of CO2 by heat absorption. The CO2-absorbing solution 1005 from which part of CO2 is released inside the regeneration tower 1007 is called “semi-lean solution”. The semi-lean solution (not shown) becomes the lean solution 1008 from which most of CO2 has been removed when reaching the bottom of the regeneration tower 1007. The lean solution 1008 is supplied to the absorption tower 1004 through a lean-solution supply line 1018. Further, the lean solution 1008 is heated by saturated steam 1021 in a regeneration heater 1020, and the saturated steam 1021 used in the regeneration heater 1020 is released as steam condensed water 1022.
Meanwhile, CO2 gas 1024 with steam is released from the top portion of the regeneration tower 1007, the steam is condensed by a condenser 1025, and water is separated by a separation drum 1026, and then CO2 gas 1027 is released to the outside of the system and is recovered. Water 1028 separated by the separation drum 1026 is supplied to the upper portion of the regeneration tower 1007 and to the top portion of the second washing unit 1013-2 through a water circulation line 1029 for recovering the CO2-absorbing solution, and is used as water 1030 for recovering the CO2-absorbing solution.
It is noted that the CO2 recovery system 1000 in FIG. 8 can be retrofitted to an existing exhaust gas source to recover CO2 therefrom, and can be simultaneously provided to a new exhaust gas source.
The conventional CO2 recovery system 1000 is used in gas-fired boilers so far, however, it is recently used also in coal-fired boilers. Hereafter, if the conventional CO2 recovery system 1000 is used in the coal-fired boiler, a greater amount of solid content containing soot and fly ash (coal ash) which cannot be removed by a desulfurization unit (not shown) or the like is discharged than that of the gas-fired boiler.
The solid content is removed in the absorption tower 1004, however, the solid content is gradually accumulated in the lean solution 1008. Therefore, if the conventional CO2 recovery system 1000 is used in the coal-fired boiler, it is necessary to provide a filtration membrane apparatus that removes the solid content in the CO2-absorbing solution.
The solid content in the CO2-absorbing solution 1005 is collected or removed by an element of a filter or a filtering material of the filtration membrane apparatus. A cartridge filter, a precoat filter, or the like is used as the filter.
However, because the CO2-absorbing solution is attached to the filter, if the filter or the filtering material is discarded as it is upon replacement thereof, the loss of the CO2-absorbing solution 1005 increases.
Moreover, if water is used to clean the CO2-absorbing solution attached to the filter and the water is discharged as wastewater, the wastewater contains the CO2-absorbing solution 1005 such as amine. Because of this, a chemical oxygen demand (COD) value in the wastewater is high and the water cannot thereby be discharged to the outside of the system as it is as wastewater.
Besides, the CO2-absorbing solution 1005 attached to the filter is cleaned with water supplied from the outside, the concentration of amine in the CO2-absorbing solution 1005 decreases.
Therefore, it is desired to develop a CO2 recovery system that can be operated similarly to the case of using the conventional CO2 recovery system 1000 in the gas-fired boiler and can continuously remove much of the solid content contained in the lean solution 1008 even if the CO2 recovery system is used in the coal-fired boiler and the amount of soot greatly increases.